Standard freighters require 4 components - Armor Plates, Cargo Bay, Construction Parts and Propulsion Engine in addition to the Hull. BPOs are seeded at NPC stations throughout empire space. Prices are roughly 1.5 billion ISK each and it will take 6 months at a bonused facility to research them to a competitive ME/TE level. You can buy researched BPO’s on contract at somewhat higher prices.
If you want to build freighters, BPCs are available on contract and, as already mentioned, you can also buy BPC kits for convenience. To build freighters profitably you’ll need access to a lowsec engineering complex with a Thukker Capital Component Manufacturing Material Efficiency rig which gives a 7% material efficiency bonus. If you’re building in highsec, it may be cheaper to buy the components than it is to build them.